The inventions concerns a device and method to provide quality control for a photosensor, especially a photodiode array or a photodiode matrix, whose output signal depends on the intensity of an input signal formed by electromagnetic waves, whereby the photosensor to be tested receives stimulation signals forming the input signals while the stimulation signal intensity of the stimulation signals is varied, and whereby the associated output signals of the photosensor to be tested are measured and recorded for evaluation purposes.
Photosensors, especially photodiode arrays, are for example used in spectrometers, color measuring systems, scanners and pattern recognition systems. A photodiode array is formed by a linear arrangement of photosensitive elements, and it generates output signals that are a measure of the light intensity received by the individual photosensitive elements. Numerous different types of such photodiode arrays are obtainable. Simple types consist of just one arrangement of a number of photodiodes with either conventional anodes or cathodes, whereby the individual photodiodes are connected by separate terminal posts for the output signals. To improve the signal precision and/or improve the usefulness or user suitability, the photodiode arrays on chips or wafers have been developed into integrated photodiode arrays with greater functionality. Of particular interest are xe2x80x9cactive pixel sensorsxe2x80x9d (APS). Their particular feature is that increasingly complex circuit elements are assigned to each pixel (i.e., each photosensor element). Such active circuit elements can have functions that range from amplification to digitalizing circuit functions. Two main types are differentiated: multiplex photodiode arrays and parallel photodiode arrays.
With multiplex photodiode arrays, specific electronic circuits are integrated on the chip that permit the photosignals to be read out and processed via a single signal path. Depending on the functionality created on the chip, the output signals are either analog or digital. The charges generated by the individual photodiodes are stored between the readout cycles in the connected capacitors via an integration interval. At the end of this integration interval, pixels are read out when the respective charges are transmitted sequentially to a common line via electronic switches. In this case, an A/D converter in the additional signal path is foreseen that sequentially converts the signals into digital values. Frequently, such A/D converters are already included in the silicon chip. Usually xe2x80x9csuccessive approximationxe2x80x9d-type converters are used, since they represent a favorable compromise between speed and complexity. Unfortunately with this type of converter, procedural variations frequently lead to differential non-linearity.
With parallel photodiode arrays, analog/digital converters for each photoelement are on the chips. The parallel photodiode arrays provide simultaneously generated digital output signals that are accessible either via serial or parallel bus systems. Thus such a sensor offers simultaneous operation which is very important in many applications. However, the complexity of this sensor is greater, and the frequency of defects accordingly increases during the manufacturing process.
In addition to photodiode arrays with a linear arrangement of photosensors, prior-art matrix photodiode arrays also exist. These have a flat structure, and the pixels are in rows and columns. Frequently, the sensors are used as camera chips or image sensors.
Increasing the functionality on the photosensor chips has made their design, handling and function increasingly complex. This also increases the probability of defects during the manufacturing process. Specialized tests must therefore be carried out to check the properties of such photosensors during the manufacturing process for quality control. This requires highly specialized optical stimulation signals and highly specialized test methods that satisfy the requirements for precision and test time. Typical photosensor properties to be tested and characterized are the input/output function concern integral and differential linearity, the response function or output signals as a function of the wave lengths, the noise or disturbance intensity as a function of the signal intensity of the input signals, and the uniformity or homogeneity of the response signals of each photoelement in the photodiode array. In addition, the mechanical limitations need to be considered, especially the accessibility of the photosensors during tests on the wafer level carried out during manufacture.
The input/output transmission function must be tested to provide quality control of such photosensors. An optical stimulus signal is generated that is applied to the sensor whose response or output signal is measured with varied input signal intensity. The respective stimulation signal intensity must be known in this context. This is determined with the aid of a reference sensor whose input/output transmission function is known. Normally a measurement is first made using the reference sensor that is mounted at the device position at which the photosensor to be tested will later be mounted.
As a first step, it is therefore necessary to measure and characterize the stimulation signal using the reference sensor. Then in a second step, the reference sensor must be removed and replaced with the photosensor to be tested (DUT). Then another measurement must be made using the same stimulation signal in regard to its time characteristic and intensity under the same test and environmental conditions. By measuring and recording the respective output signals for evaluation purposes, the results of both measurements can be compared which serves as a measure for the quality of the photosensor to be tested. It is necessary to make the two measurements (of the setpoints using the reference sensor and of the actual values using the photosensor to be tested (DUT)) one right after the other or, in any case, separated only by a brief interval. Otherwise, substantial uncertainty can sometimes arise from the intermediate change in environmental conditions or changed radiation pattern of the light source, for example due to aging over time. The calibration measurement therefore will also have to be repeated at certain intervals using the reference sensor, and the reference sensor and sensor to be tested (DUT) will have to be mounted and removed each time.
This conventional state of the art procedure has a series of problems and difficulties. Changing the sensor once or several times causes uncertainty from altering the precise three-dimensional position of the sensor that can produce corresponding measurement uncertainty or imprecision. Further uncertainty can arise from differences in the respective types of photosensors, i.e., the reference sensor and the sensors to be tested, and in the properties of the output signals and the measuring setup. The previously required installation and removal procedure leads to long test cycles and allows little or no automation in quality control.
In particular when measuring small changes in the stimulation signal intensity, for example in photometric measuring procedures using spectrometers, absorbance detectors, color measuring systems or in the case of absorption measurements, faults in differential linearity of the photosensors have a disturbing influence and are characterized by the differential linearity (DNL) of the photosensor to be tested. Such faults distort the measuring results and increase the quantization noise. DNL has been determined to date by making a small continuous change in the input signal and measuring the associated output signals.
In the case of photosensors with digital output signals, DNL is defined as the deviation of the individual quantization steps from the average expected quantization step. This DNL can arise in the form of xe2x80x9cmissing codesxe2x80x9d (FIG. 3), xe2x80x9cdouble codesxe2x80x9d (FIG. 4), or as a xe2x80x9cdead zonexe2x80x9d (FIG. 5). In contrast, FIG. 6 shows a fault-free input/output signal transmission function of a photosensor.
The DNL can be determined by comparing or referring the input signal to the output signal of the photosensor. To date, this has required the precise knowledge of the size of the input signal with a targeted control of the entire control range or else the size of the output signal must be determined by reference testing. Both procedures are very involved and time-intensive. It is very difficult to obtain the desired amplitude resolution, precision and stability of optical control signals (drift, temperature, local differences, etc.), and the insufficiency of optical reference formation is sometimes substantial. In the latter case, design-related features, differences in the characteristic of the sensor to be tested and of the reference sensor, and statistical uncertainty can have a very strong influence on the result. This is all the more the case as the sensor increases in size.
The problem of the invention is therefore to create a quality control method for photosensors and a device to carry out this method that allows more precise test results with shorter cycles and a high degree of automation, and allows the tests to be carried out directly at the wafer level.
According to one aspect of the invention, the photosensor to be tested receives at least two independently controllable superposed individual optical stimulation signals with individual optical stimulation signal intensities. This makes use of the basic concept of the linear superposition of the optical stimulation signal intensities of the individual optical stimulation signals. This makes it possible to eliminate the involved calibration procedure with the frequent mounting and removal of a reference sensor and exchange with the photosensor to be tested which yields uncertainty. This allows more precise test results in quality control of the photosensor to be tested and permits quick and economical tests on the wafer level and a high degree of automated quality control. It is particularly advantageous that instead of the cyclical reference measurements, the transmission characteristic of the optical system only has to be calibrated once, and the luminous flux and its change can be measured and evaluated online.
It is particularly advantageous when the individual stimulation signals are superposed in an integrating sphere that preferably has at least one outlet opening for the superposed, individual stimulation signals. When such a light-integrating sphere is used, the individual stimulation signals can be advantageously superposed due to their highly diffuse reflection on the inner wall of the sphere, and they are available there or preferably at (at least) one exit opening to be applied to the photosensor to be tested.
In a first alternative solution, the integrating sphere is optically connected to a reference photosensor for detecting an overall stimulation signal flow created by superposing the individual stimulation signals and assigned to the resulting nominal stimulation signal intensity, whereby the nominal stimulation signal intensity is measured and detected for evaluation purposes with the reference photosensor preferably at the same time as the output signals of the photosensor, whereby their relationship to each other is a measure of the quality of the photosensor to be tested.
In another alternative solution for the photosensor quality control method, the nominal output signals of the photosensor to be tested corresponding to the superposed stimulation signal intensities are calculated using the measured output signals of the photosensor. Their relationship to the measured output signals serves as a measure of the quality of the tested photosensor. In particular, this method makes it very easy to effectively check the integral linearity (INL) of the tested photosensor and the uniformity or homogeneity of the response function of the individual photoelements in a photodiode array.
This is accomplished in a particularly advantageous manner by sequentially measuring the photosensor output signals assigned to possible combinations of individual stimulation signals and recording them for evaluation purposes. An algorithm is used based on multiple linear regression to determine the probable individual stimulation signal intensities Coefx actually applied to the photosensor and assigned to the individual stimulation signals according to the following equation:
xe2x80x83R=R0+Ctl0,i*Coef0+Ctl1,i*Coef1+Ctl2,i*Coef2+ . . . +Ctln,i*Coefn
whereby R is the output signal of the photosensor, R0 is the also calculated zero point deviation, Coefx is the intensity of the coefficient representing the individual stimulation signal x, and Ctrlx,i is a control matrix for the individual stimulation signal x, and where the nominal output signals of the photosensor to be tested corresponding to the superposed individual stimulation signals are then calculated using the equation for all possible combinations of individual stimulation signals, whereby their relationship to the output signals measured at the same stimulation signal intensities is a measure of the linearity of the photosensor to be tested, and the nominal linear signal transmission characteristic of the photosensor to be tested can accordingly be determined.
A particularly meaningful and easy measure of the linearity of the photosensor to be tested can be obtained by calculating an X vector assignable to a Cartesian coordinate system as the quotient from the nominal output signals for a specific combination of individual stimulation signals and the nominal output signals for the combination at which all stimulation signals are superposed, and a Y vector is assigned to the X vector that corresponds to the measured output signals of the photosensor to be tested for the same specific combination of individual stimulation signals, and a line slope and an axis section are calculated from this as a regression parameter by means of linear regression, and the axis section and line slope serve as a measure for the nominal linear signal transmission characteristic of the photosensor to be tested, and the characteristic can be determined solely using two parameters, i.e. the line slope and the axis section.
The above-described procedure for photosensor quality control to determine the INL can also be used to determine the DNL, although the attainable resolution is limited. However, this simple and fast procedure can be used to check the general function of the photosensor and can serve to initially segregate defective sensors. The remaining photosensors can then be checked in a different procedure for potential differential non-linearity.
Accordingly, the problem according to an alternative solution is also solved in that the photosensor has several individual photosensors that provide output signals and are supplied with stimulation signals with an essentially constant, relative, local stimulation and constant, relative, temporal stimulation signal intensity distribution which is continuously changed. In particular, this opens up favorable possibilities for detecting and determining differential non-linearity of such photosensors without requiring a precise knowledge of the level of the output signals or corresponding reference measurements. There can be any kind of local distribution of the stimulation signal intensity over all photosensor elements or pixels, however it should be approximately homogenous to create similar control conditions for all photosensor elements.
For practicality purposes all photosensor elements are simultaneously supplied with the stimulation signals which results in corresponding time advantages with a high degree of automation.
The photosensor elements are advantageously supplied with stimulation signals whose intensity distribution differs locally across the photosensor elements. It is assumed that the individual photosensor elements are controlled similarly but not the same so that potential, level-related errors do not arise simultaneously for all pixels. In this manner, one can discern whether the faults are a result of a faulty input signal or from a defect of the photosensor to be tested.
It is particularly advantageous when the output signals of each photosensor element are measured as offset signals and the associated offset signal values are detected without supplying the stimulation signals; it is also advantageous when the output signals of each photosensor element fed stimulation signals are measured as measuring signals and the associated measuring signal values are determined, and when the respective offset signal values are subtracted from the measuring signal values of each photosensor to provide an offset correction, and the resulting local and time-related nominal output signal values are determined, and when the local and time-related nominal output signal values are then standardized with a location-related intensity value determined from these nominal output signal values, and when a time-related intensity value determined according to a statistical procedure from these data values is subtracted from the resulting location and time-related data values. In that way, taking into account the remaining noise, the deviations from, e.g. established specific threshold values which exceed upper or lower limits due to differential non-linearities, can be determined with regard to position and magnitude for each photosensor element. This procedure takes advantage of statistical and probability calculations, since deviations from normal behavior may be effectively displayed during testing of photosensors with a majority of elements of the same type. The positional and temporal progression of intensities may be compiled from redundant information, so that this information need not be obtained using another more expensive and/or error-prone method.
Because of increased statistical accuracy, it is worthwhile to perform the above steps in the order specified, since generally fewer positional measurement data related to the photosensor are present than temporal data. This means, however, that these steps may also be performed in reverse order.
It is useful that the position-related intensity value for each photosensor element (71, 72, 73) be calculated from the average value of the nominal output signal values of each photosensor element over all measurement periods, and that the time-related intensity value be calculated for each measurement period from the median value of the resulting position- and time-related data values of each measurement period. Use of the average value as a reference value has proved useful for the calculation of the position-related intensity value since it is to be assumed that non-linearity errors arise simultaneously only for individual photosensor elements. The average value, the median value, or the reporting of the maximum from the frequency distribution of the measurement values of all position-related intensity values for any point in time involved are used as reference values for calculation of the time-related intensity value, and for the ideal case in which no differential non-linearity errors arise, these methods all provide the same result. However, for the case in which the measurement data are overlaid by differential non-linearity errors, considerable differences may result so that the selection of the suitable procedure is of high significance. Formation of the average value is not suitable here since many errors may have strong influence on the average value, thus rendering the result inaccurate. The best result is obtained by use of the maximum from the determination of frequency distribution, since it may be assumed that, because of the monitored production procedure, the frequency of data not affected by error exceeds all other frequencies, or that grave defects were detected in the foreground by a simpler testing procedure. This calculation is, however also the most involved. It has been shown that the use of the median value represents a good compromise, since in practice, formation of the median value approximates the determination of the frequency distribution.
For photosensors that produce digital output signals with specified quantification stages, it is useful to select the temporal alteration of the stimulation signal intensity such that a number, or a majority, of measurement values are compiled for each quantification stage. In this manner, a filtration serving to suppress the measurement data noise level followed by a measurement data reduction is possible in order to enable achievement of accurate test results during relatively brief testing cycle times. In order to reduce statistical unreliability, a suitably high, preferably larger number of measurement values would be compiled than would be reported during the measurement of the offset signals from the photosensor elements (71, 72, 73).
Filtration of output signals such as using an FIR- or IIR filter might be performed to reduce and minimize statistical unreliability or deviations, especially the noise level of the measurement values, and the resulting data values thus be determined. For this, the filtering would be conducted in such a manner that regular errors, i.e., inconsistencies in the input/output conversion function of the photosensor being tested caused by differential non-linearities, are maintained. A reduction of data values, preferably to four data values per quantification stage, would be then performed in order to reduce the quantity of data and the testing duration.
According to an advantageous embodiment of the method, the photosensor elements of the photosensor to be tested would be subjected to at least two independent, controllable, overlapping individual stimulation signals with individual stimulation signal intensity.
It would be particularly advantageous if the individual stimulation signals are superimposed into an integrated sphere that preferably includes at least one exit opening for the superimposed individual stimulation signals. In this manner, a favorable opportunity for data preparation for the determination of the DNL is achieved with respect to the positional and temporal progression of stimulation intensities.
For this, it is advantageous if at least the first stimulation signal of the independently-controlled stimulation signals constantly varies throughout the period, and is superimposed on a further individual stimulation signal, whereby the second stimulation signal is transferred from the first switched condition (preferably with zero signal intensity) into a second switched condition with a greater signal intensity. Then the generation of the constantly-varying stimulation signal intensity may be divided into several sections so that additional stimulation signals switchable in stages may be further superimposed onto the constantly-varying stimulation signals. In this manner, the resolution requirements for the constantly-varying stimulation signals may be reduced so that good resolution of the stimulation signal source producing the first stimulation signal is possible in any range. For this, it is immaterial in which stage and in which sequence the switchable stimulation signals are engaged. Further, the measurement need not be temporally connected, but rather can be interrupted from section to section. It is further immaterial whether the variable stimulation signal is constantly increasing or constantly decreasing as it is superimposed.
It would be advantageous if the individual stimulation signals are each produced by a stimulation signal source that includes a control range of possible stimulation signals intensities, and whereby the control range of the first stimulation signal source is greater than the control range of the second stimulation signal source, so that their control ranges overlap. When conducting a procedure with increasing stimulation signal intensity, the control range of the second signal source will overlap at the top, or during a procedure with decreasing stimulation signal intensity, the control range of the second signal source will overlap at the bottom. In this manner it is ensured that no unsteady location is mistakenly induced that otherwise might be interpreted as a photosensor defect.
The device according to the invention for the photosensor quality control method has a stimulation signal source to generate stimulation signals supplying the photosensor that can be adjusted to have different stimulation signal intensities, and a controller coupled to the stimulation signal source to adjust the different stimulation signal intensities, and a measured data detector that can be connected to the photosensor to be tested to measure and detect the output signals of the photosensor, whereby the stimulation signals consist of at least two independently controllable, superposed individual stimulation signals with individual intensities. A highly specialized stimulation signal can be provided for direct quality control of the photosensors to be tested. This device can provide highly automated quality assurance of the photosensors to be tested with tests on the wafer level that yield much less measuring uncertainty than prior test arrangements for photosensor quality assurance. Correspondingly more precise test results can be attained.
It is useful to provide at least two independently controllable stimulation signal sources to generate the individual stimulation signals. This allows a particularly easy and economical test setup.
The stimulation signal source(s) is/are usefully designed with LEDs, whereby each LED preferably emits essentially the same wavelength. Accordingly, a unit consisting of several LEDs can be advantageously created. To check the spectral response function of the tested photosensors, either several such units can be provided with several LEDs, or several subunits with several LEDs can be provided in the unit with LEDs, whereby the LEDs in the individual units or subunits emit light at a specific, i.e., essentially equivalent wavelength. The units or subunits can usefully be designed with red, yellow, green and/or blue LEDs.
It is particularly advantageous when the stimulation signal source is connected to a sphere integrating the individual stimulation signals that preferably has at least one outlet opening for the superposed individual stimulation signals. The integrating sphere is usefully designed as a hollow sphere and has an inner coating with a nearly perfectly diffuse or Lambert-type material, and it is characterized by a particularly strong reflection. Such an integrating sphere allows the individual stimulation signals to be homogeneously superposed in a particularly effective manner due to the highly-diffuse reflection characteristic of the inner wall of the sphere and allows the photosensor to be tested to receive the individual stimulation signals superposed in the integrating sphere in a correspondingly effective manner.
The integrating sphere is advantageously optically connected to a reference photosensor to detect the overall stimulation signal flow formed by superposing the individual stimulation signals and assigned to the resulting stimulation signal intensity. The reference photosensor is arranged so that it only receives the individual superposed stimulation signals preferably reflected only by the integrating sphere, i.e., not directly supplied with the individual stimulation signals. By using this reference photosensor, one can check whether the desired linear relationship of the superposed individual stimulation signals or their stimulation signal intensities is maintained. In addition, the reference photosensor can be used to measure the nominal stimulation signal intensity and relate it to the preferably simultaneously measured output signals of the photosensor to be tested and hence obtain a measure of the quality of the photosensor to be tested. The reference photosensor and the photosensor to be tested hence do not have to be repeatedly mounted and removed as required by the state of the art. However, this type of quality control requires several individual measurements so that the measuring results can still have a relatively large degree of statistical uncertainty.
The integrating sphere is usefully optically connected to at least one optical fiber to receive the superposed individual stimulation signals and send them to the photosensor to be tested. This allows the location where the stimulation signals are generated to be spatially separate from the actual site where the photosensor is measured during manufacture.
It is also useful when a beam former, preferably an optical positive lens, is provided to focus the superposed individual stimulation signals on the surface of the photosensor to be tested. The beam former can be directly connected to the end of the optical fiber that has the photosensor to be tested, or it can be placed between this end of the optical fiber and the photosensor to be tested.
The above measures together and by themselves promote more precise test results with shorter test cycles and with a higher degree of automation, and allow the tests to be carried out directly on the wafer level.
Other features, perspectives and advantages of the invention can be found in the following description that refers to a figure.